Using regular expressions to find a word with the five letters abcde, each letter appearing exactly once, in any order, with no breaks in between

Question

For example, the word debacle would work because of debac, but seabed would not work because: 1. there is no c in any 5-character sequence tha

Answer 1

/
   (?= .{0,4}a )
   (?= .{0,4}b )
   (?= .{0,4}c )
   (?= .{0,4}d )
   (?= .{0,4}e )
/xs

---

It's probably results in faster matching to generate a pattern from all combinations.

use Algorithm::Loops qw( NextPermute );
my @pats;
my @chars = 'a'..'e';
do { push @pats, quotemeta join '', @chars; } while NextPermute(@chars);
my $re = join '|', @pats;

abcde|abced|abdce|abdec|abecd|abedc|acbde|acbed|acdbe|acdeb|acebd|acedb|adbce|adbec|adcbe|adceb|adebc|adecb|aebcd|aebdc|aecbd|aecdb|aedbc|aedcb|bacde|baced|badce|badec|baecd|baedc|bcade|bcaed|bcdae|bcdea|bcead|bceda|bdace|bdaec|bdcae|bdcea|bdeac|bdeca|beacd|beadc|becad|becda|bedac|bedca|cabde|cabed|cadbe|cadeb|caebd|caedb|cbade|cbaed|cbdae|cbdea|cbead|cbeda|cdabe|cdaeb|cdbae|cdbea|cdeab|cdeba|ceabd|ceadb|cebad|cebda|cedab|cedba|dabce|dabec|dacbe|daceb|daebc|daecb|dbace|dbaec|dbcae|dbcea|dbeac|dbeca|dcabe|dcaeb|dcbae|dcbea|dceab|dceba|deabc|deacb|debac|debca|decab|decba|eabcd|eabdc|eacbd|eacdb|eadbc|eadcb|ebacd|ebadc|ebcad|ebcda|ebdac|ebdca|ecabd|ecadb|ecbad|ecbda|ecdab|ecdba|edabc|edacb|edbac|edbca|edcab|edcba

(This will get optimised into a trie in Perl 5.10+. Before 5.10, use Regexp::List.)

Answer 2

#! perl -lw
for (qw(debacle seabed feedback)) {
    print if /([a-e])(?!\1)
        ([a-e])(?!\1)(?!\2)
        ([a-e])(?!\1)(?!\2)(?!\3)
        ([a-e])(?!\1)(?!\2)(?!\3)(?!\4)
        ([a-e])/x;
}

Answer 3

Your solution is clever but unfortunately [a-e^...] doesn't work, as you found. I don't believe there is a way to mix regular and negated character classes. I can think of a workaround using lookaheads though:

    /(([a-e])(?!\2)([a-e])(?!\2)(?!\3)([a-e])(?!\2)(?!\3)(?!\4])([a-e])(?!\2)(?!\3)(?!\4])(?!\5)([a-e]))/

See it here: http://rubular.com/r/6pFrJe78b6.

UPDATE: Mob points out in the comments below, that alternation can be used to compact the above:

    /(([a-e])(?!\2)([a-e])(?!\2|\3)([a-e])(?!\2|\3|\4])([a-e])(?!\2|\3|\4|\5)([a-e]))/

The new demo: http://rubular.com/r/UUS7mrz6Ze.